Recently, a detailed study of residual stresses on the as-processed SCS-6/Ti-24Al-11Nb [0]8 composite, and SCS-6/Beta-21S composites in unidirectional [0]4, cross-ply [0/90]s, and quasi-isotropic [0/±45/90]s layups has been completed. In this study, residual stresses have been measured using X-ray diffraction (sin2ψ) technique. We have shown that the use of conventional unit cell models consisting of a quarter fiber surrounded by the matrix material to predict residual stresses for verification of experimental results is inadequate. Such models have predicted successfully the stresses at the fiber-matrix interface. However, experimental work to measure residual stresses have always been on surfaces far away from the interface region.

In this paper, the approach taken in extending the conventional unit cell model to the concept of multifiber models to predict average stresses are presented. In this process, several modeling issues have been identified. These issues are (1) use of conventional unit models for prediction of average surface residual stresses, (2) effect of orientation of the sub-surface plies on the residual stresses in the surface ply, (3) residual stresses in the interior plies, and (4) constituent material properties.